In pressure sensor technology, absolute pressure sensors, differential pressure sensors and gauge pressure sensors are commonly known. Absolute pressure sensors determine the prevailing pressure in an absolute sense, i.e. relative to a vacuum, while differential pressure sensors determine the difference between two separate pressures. With gauge pressure sensors, the measured pressure is determined relative to a reference pressure supply, wherein the ambient, prevailing atmospheric pressure serves as this reference pressure supply. This sort of gauge pressure sensor normally comprises a closed measurement chamber with a pressure sensitive diaphragm, which seals the measurement chamber on one side. The pressure, which is to be measured, acts upon the outside surface of the sensing diaphragm. By way of example, the measurement chamber is connected to the surrounding environment via a vented tube through which the ambient air enters the measurement chamber so that the ambient pressure exists in the measurement chamber as well as on the inner surface of the sensing diaphragm. The sensing diaphragm bends, depending on the existing gauge pressure. This bending is converted into an electrical signal, by means of an electromechanical transducer, and this is used to determine the pressure of the medium relative to the reference pressure supply.
To produce the electrical signals, which mirror the degree of the bend of the sensing diaphragm, the sensing diaphragm is normally provided with a first electrode and the opposite surface of the measurement chamber is provided with a second electrode and a third electrode. The second electrode serves as a reference electrode and is so arranged so that its capacitance is only influenced by the temperature, humidity and other work environment specific parameters and not from the bending of the sensing diaphragm. The third electrode composes, along with the first electrode, a precision capacitor, which is especially sensitive to the bending of the sensing diaphragm. The capacitance of a capacitor composed in this way changes as a function of the bending of the sensing diaphragm, so that the capacitance of the gauge pressure is determinable.
A further group of commonly known gauge pressure sensors comprises a sensing diaphragm, on which strain sensitive elements such as strain sensitive films are arranged. The resistance of the strain sensitive films is dependent on the bending of the sensing diaphragm and so serves as a measurable quantity for the evaluation of the pressure.
A further group of commonly known gauge pressure sensors, from the prior art, comprises a flexural plate, whereon pressure sensitive elements are arranged. This flexural plate is not directly exposed to the process pressure, but is in contact with the process via a pressure transmitter. The pressure transmitter can be a large bulk solid or pipe filled with an incompressible fluid, e.g. oil, which is, vis a vis the process, sealed via a pressure sensitive diaphragm.
A general characteristic of the described sensors is a diaphragm, which bends depending on the difference in the pressure on both sides. This bending is converted into an electrical signal via various methods (capacitive or piezo-resistive).
Gauge pressure sensors are quite frequently employed in industrial processes, where they are exposed to high temperature changes and to high temperature differences between the process and the surrounding environment. During a sudden cooling, the dew-point, of the air in the space inside the sensor, can be reached, leading to condensation of the air moisture on the cold parts of the sensor. Humid air can end up in the sensor as a particular result of the opening to the surrounding environment, which is necessary to supply the measuring element with the reference pressure supply.
Electromechanical transducers as well as sensor electronics are typically very sensitive to moisture. In order to complete a reliable pressure calculation, the access of moisture condensate into the inside of the gauge pressure sensor must be safeguarded against. According to EP 1070948 A1 there is a gauge pressure sensor, wherein the reference pressure supply tube comprises a segment of enlarged cross-sectional area, wherein there is a hydrophilic unit. This is intended to remove water from moist air, so that only dry air accesses the measurement chamber. However, the disadvantage here is that with a high flow rate of air through the hydrophilic unit, a satisfactory drying of the air is not guaranteed. Such an increased flow rate can especially occur in with a significant drop in temperature, because this leads to a large compression of the air in the reference pressure supply tube.